To have optimum engine combustion, the spark timing of an engine must be precisely controlled. On a typical four cylinder, 1,400 cc gasoline engine with a compression ratio of 9:1, the combustion position at which the combustion of fuel can produce the maximum power output is when the crank is at 12-degrees at the After-Top Dead-Center (ATDC). If the sparking timing is controlled to allow the combustion to occur at the above ATDC, maximum power output will be produced at a fixed amount of fuel consumption regardless of the variations in the gasoline octane rating, engine speed, and load.
When the mixed gas in the engine combustion chamber is ignited, the combustion spreads outwardly from the ignition point after an inherent delay time. The delay time is primarily dependent upon the fuel octane value, the mix ratio of the fuel, and the compression ratio. The delay time does not vary with the changing of the engine speed and load. Therefore, the rotation angle of the crank determines the proper angle of the spark advance.
Conventional spark timing advance systems are comprised of a weight advancer and a vacuum advancer. The weight advancer functions by a centrifugal force and a spring. When the engine rotation speed increases, the weight is displaced outwardly by the centrifugal force and thus advances the spark timing. The vacuum advancer is comprised of a vacuum chamber having on one side a diaphragm on which a lever is connected. When the diaphragm moves, the lever moves the brake pointer plate in compliance with the vacuum pressure, and thus advances the spark timing.
A conventional vacuum advancer is shown in FIG. 1. These conventional advancers are designed to advance the spark by using a combination of engine speed and vacuum. Generally, these advancers are not able to cause combustion to occur at a position of maximum efficiency under all driving conditions. Therefore, much energy is wasted. A characteristic graph of all the driving conditions is shown in FIG. 5. The figure indicates that proper advancing is not achieved with conventional systems and that an improvement is needed.
Repeated experiments conducted by the inventor indicates that the graph should form a straight line as shown in FIG. 6. In FIG. 7, three curves a, b and c are shown. The curve "a" illustrates the most suitable spark timing where combustion occurs at maximum efficiency. The curves "b" and "c" illustrate two cases of a delayed spark timing. In both of these curves, the delivered power output is less than that provided by curve "a".